Indoor air quality is extremely important for productivity and occupant health in any industrial, commercial, or residential property.
HVAC air conveyance systems, with associated ductwork and ductless mini-split systems, provide acceptable indoor air quality atmospheres. However, they must be routinely inspected, maintained, and cleaned to operate most efficiently and cost effectively. This routine includes cleaning the interior of the ductwork and replacing filters.
Air conveyance systems filter out dust, particulates, volatile organic compounds (VOCs), mold spores, bacteria, and viruses. These systems also lower humidity levels while removing moisture levels in a building. Unfortunately, dust, particulates, VOCs, mold spores, bacteria, and viruses can accumulate on the interior surfaces of air conveyance systems and interior ductwork. This accumulated debris can be aerosolized and transmitted through air currents.
Studies on virus transmission
In regard to SARS-CoV-2 transmission, a 2020 Florida Atlantic University study showed that a human sneeze or cough can disperse smaller droplet nuclei into air currents up to 12 feet away and linger in the air for more than one minute. A study conducted in an MIT laboratory using high-speed cameras and light confirmed that a fine mist of saliva and mucous can burst from a person’s mouth at nearly 100 miles per hour and can travel as far as 27 feet.
Research suggests it is possible coronavirus droplets can be aerosolized, carried, and transmitted through HVAC duct systems.
A ducted HVAC system creates air currents, which can likely carry smaller viral particles even further. A research letter published in the U.S. Centers for Disease Control and Prevention’s (CDC) Emerging Infectious Diseases journal in July 2020 detailed a COVID-19 outbreak associated with air conditioning in a restaurant in Guangzhou, China. Ten people from three families eating in the restaurant became infected. Aerosolized droplet transmission was prompted by air-conditioned ventilation. A video from the restaurant showed many of these patrons were sitting more than six feet apart from one another. Researchers concluded strong airflow from the air conditioner could have propagated droplets from one table to another and back.
Research published on nature.com in July 2020 detailed an ongoing study of environmental contamination that obtained surface and air samples in two Nebraska Biocontainment Units (NBUs) and nine residential isolation rooms housing individuals who tested positive for SARS-CoV-2. Room surfaces tested included ventilation grates, tabletops, and window ledges. Virus samples with the highest concentration were recovered from an air-handling grate in the NBU.
Airflow in the NBU suites originated from a register at the top center of the room and exited from grates near the head of the patient’s bed on either side of the room. Airflow modeling suggested that some fraction of the airflow was directed under the patient’s bed, which may have caused the observed contamination under the bed, while the dominant airflow likely carried particles away from the patient’s bed toward the edges of the room, passing by the windows, resulting in some deposition there.
The New England Journal of Medicine published a letter to the editor in March 2020 from a group of researchers who studied the stability of SARS-CoV-2 in aerosols and on various surfaces. Their research indicated that infectious aerosol may persist for several hours and on surfaces for as long as two days. The most prolonged viability of viruses was on stainless steel and plastic. The estimated median half-life of SARS-CoV-2 was approximately 5.6 hours on stainless steel and 6.8 hours on plastic. In conjunction, influenza viruses can spread through the air on dust, fibers, and other microscopic particles, according to research published in August 2020 from the University of California, Davis and the Icahn School of Medicine at Mt. Sinai.
A look at airflow
Although ductwork interiors can be made of smooth metal, fine dust can collect on air duct surfaces, vents, motors, and coils. Flakes of dead skin, hair, and pet dander catch onto the dust, and before long, this process creates a dense, mat-like environment where viruses, bacteria, mold, dust mites, and allergens can flourish. This is similar to a vehicle’s exterior during a spring pollen season, as the pollen would coat the vehicle and windows.
For instance, when dust is carried on air currents generated by air conditioning and similar devices, the dust takes on a positive or negative static electric charge as it makes contact with various objects. Dust with a positive electric charge will be attracted to objects with a negative electric charge and vice versa. The greater the amount of dust in the air, the more considerable the amount of dust that clings to objects within the room.
Also, if sources of dust (mainly people and clothing) are electrically charged, the dust that is generated from these sources is electrically charged as well. This attractive force generated by static electricity is known as the “Coulomb force.” Also, ductwork return airstreams contain levels of humidity and moisture. As a result, this moisture can adhere to the ductwork interior and capture dust particulate.
HVAC air filters might help in capturing the viral particles and dust particulates. A high-efficiency particulate air (HEPA) filter effectively removes small particles, 99.7% of particles sized 0.3 microns. A minimum efficiency reporting value (MERV) filter removes larger particles, 0.3 microns to 10 microns. A MERV filter rated 17 or higher is considered HEPA-like in efficiency. However, most commercial buildings have MERV filters rated 12 or lower. Unfortunately, a coronavirus particle is about 0.1 microns and cannot be viewed with human eyesight. So, there’s no guarantee that a HEPA filter would stop viral particles and dust particulates.
Current standard air handling units (AHUs) in HVAC systems circulate up to 80 to 90% of the air in regular systems during peak-load conditions such as in winter and summer when outdoor ventilation airflow is set at the minimum percentage to save energy. Standard filtration units in HVAC systems cannot remove the virus within an airstream effectively. The HVAC system has become a central point of contact to spread the virus by recirculating contaminated air into space.
Based on the cited research, inspection, testing, surface and air sampling, cleaning, and disinfecting HVAC air conveyance systems and associated ductwork can help prevent the transmission of SARS-CoV-2.
HVAC system and ductwork cleaning and disinfecting best practices
- Review HVAC air conveyance systems and associated ductwork drawings and diagrams.
- While donning proper personal protective equipment (PPE), perform a visual inspection of the exterior and interior of the HVAC air conveyance system and associated ductwork, registers/vents, motors, coils, and filters. Use a video borescope camera if necessary, and take digital pictures. Document and log any observations, even if the ductwork “looks clean.”
- Test and inspect for any air leakage, VOCs, particulates, and metal seam issues on ducts.
- Perform pre- and post-surface swab sampling of the HVAC air conveyance systems and associated ductwork, registers/vents, motors, coils, and filters. The pre-surface swab sampling may identify any hazardous substances and hazardous particulates.
- Perform pre- and post-air sampling of the project area. These samplings will help identify and isolate specific systems and affected areas, along with any hazardous substances and/or particulates.
- Use environmental controls and infection control risk assessment protocols to contain and isolate the cleaning and disinfecting project. This includes setting up HEPA air scrubbers, negative air machines, and containment barriers while using a particulate counter and air pressure monitor.
- Implement ultra-low volume (ULV) fogging using a non-acidic, non-chlorine-based U.S. Environmental Protection Agency (EPA)-registered virucide, biocide, and fungicide disinfectant outside and inside ductwork, on registers/vents, motors, coils, and the entire project area, to inactivate any viral aerosols. For efficacy, allow a minimum 10-minute dwell time on surfaces or follow the dwell time per the virucide label.
- Clean the interior ductwork surfaces from larger dust and particulates, using HEPA vacuuming, rotobrushing, air whipping, and mechanical cleaning. Interior fiberglass-lined ductwork requires specialized attention and cleaning to prevent fiber damage and release. Visually inspect to prevent recontamination.
- On metal interior ductwork, registers/vents, motors, and coils, clean and disinfect using a nonacidic, nonchlorine-based, EPA-registered virucide, biocide, and fungicide disinfectant. For efficacy, allow a minimum 10-minute dwell time on surfaces or follow the dwell time per the virucide label.
- Replace and install new HEPA filters. Ductless mini-split systems may have a washable filter. It is best to vacuum (with HEPA filter) the air conveyance unit and disinfect the washable filter often with an EPA-registered virucide, biocide, and fungicide disinfectant. For efficacy, allow a minimum 10-minute dwell time on surfaces, or follow the dwell time per the virucide label.
Remember, it’s important to inspect, maintain, and clean HVAC systems on a routine schedule. If you do this, your air conveyance system will operate most efficiently, contribute to a healthier indoor air environment, and save operating costs over time.